Process to prepare camptothecin derivatives

ABSTRACT

A process is provided for the preparation of camptothecin derivatives, such as irinotecan, in a one-pot operation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improved processes to preparecamptothecin derivatives, such as irinotecan, in a one-pot operation.

2. Description of the Related Art

Camptothecin 1 (shown in FIG. 1A) is a pentacyclic alkaloid that wasisolated by Wall et al. in the early 1960s from the Chinese tree,Camptotheca acuminate (Nyssaceae). The compound raised immediateinterest as a potential cancer chemotherapeutic agent due to itsimpressive activity against a variety of tumors. However, a shortcomingof camptothecin as an anti-cancer agent was its poor solubility inwater. To overcome the solubility problem, the sodium salt wassynthesized by hydrolysis of the lactone ring. The sodium salt forms anequilibrium with the ring-closed lactone form. As its sodium salt,camptothecin was moved to clinical trials and promising activity wasinitially observed. However severe side effects and drug-relatedtoxicities finally led to discontinuation of the clinical program.

Stimulated by the challenging structure and its very interestingbiological activity, synthetic approaches to camptothecin weredeveloped. During semi-synthetic and total-synthetic chemistry programs,the particular importance of the lactone ring and the C20(S)-configuration for good biological activity was recognized. Incontrast, modifications in the A-ring and B-ring, particularly in theC9, C10 and C11 positions, were tolerated and led to improved analogues.

Second-generation camptothecin derivatives have been optimized forimproved water solubility to facilitate intravenous drug administration.Highlights resulting from various programs at different companies andinstitutions are irinotecan 2 and topotecan 3, two compounds which aresuccessfully used in clinical practice, and SN-38 4, exatecan 5,liposomal lurtotecan 6 (OSI-211) and CKD-602 7, which are in advancedstages of clinical development. The chemical structures of thesecompounds are shown in FIGS. 1A and 1B.

SN-38 is a camptothecin derivative that contains a hydroxyl group at theC10 position and an ethyl group at the C7 position. Irinotecan is acamptothecin derivative (it may also be viewed as a derivative of SN-38)that contains a sidechain at the C10 position and an ethyl group at theC7 position. Irinotecan was discovered at Yakult Honsha and was firstapproved in Japan in 1994 (Camptotesin®) for lung, cervical and ovariancancer. Today it is marketed in the U.S. by Pharmacia (Camptosar®) andby Aventis in Europe (Campto®). Irinotecan is a prodrug which is cleavedin vivo by carboxylic esterases, particularly by hCE-2, to release theactive metabolite SN-38.

The synthesis of irinotecan has been described in the chemicalliterature and in patents. A common approach to the synthesis ofirinotecan is to form SN-38 and then add a sidechain to the C10 positionof SN-38, to thereby form irinotecan. U.S. Pat. No. 4,604,463 is oneexample of a patent that describes this approach, wherein either anactivated form of the sidechain is separately formed and then reactedwith SN-38, or the C10 hydroxyl group is activated and then in aseparate reaction the sidechain is added.

Although there have been advances in the field, there remains a need forimproved methods to form irinotecan from SN-38. The present inventionaddresses this need and provides further related advantages.

BRIEF SUMMARY OF THE INVENTION

In brief, the present invention is related to improved processes toprepare camptothecin derivatives, such as irinotecan, in a one-potoperation.

In one embodiment, a process for adding a sidechain to a startingmaterial is provided comprising reacting together: (i) a compound offormula I:

wherein each of the ring atoms may be carbon, or any one, two or threeof the ring atoms may be nitrogen, and R^(a), R_(b) and R^(c) are thesame or different and independently represent one or more optionalnon-hydrogen substituents on each of rings A, B and C; (ii) an amine offormula R¹R²NH wherein R¹ and R² are the same or different andindependently represent organic groups; and (iii) phosgene or a reactiveequivalent thereof, to provide a solution comprising a compound offormula III:

In a further embodiment of the foregoing, the phosgene or reactiveequivalent thereof and the amine of formula R¹R²NH are combined toprovide an intermediate solution, and the compound of formula I is addedto the intermediate solution. In yet a further embodiment, the processfurther comprises adding pyridine or a tertiary amine to theintermediate solution along with the compound of formula I.

In another further embodiment, the process further comprises a stepwherein the solution comprising a compound of formula III is filteredand then freed from solvent to provide a residue. In more specificembodiments, the residue is purified by column chromatography,filtration, precipitation or crystallization.

In a more specific embodiment of the foregoing, the compound of formulaI is SN-38, the amine of formula R¹R²NH is piperidinopiperidine, and thecompound of formula III is irinotecan, and the process is a one-potoperation. In particular, the phosgene or reactive equivalent thereof isphosgene trihydrate, and the phosgene trihydrate andpiperidinopiperidine are combined in a solvent and allowed to reacttogether to form an intermediate solution, and the SN-38 and an organicbase are added to the intermediate solution to form the solutioncomprising irinotecan.

In further embodiments of the foregoing, the phosgene trihydrate isdissolved in a chlorinated solvent and then cooled to a temperature inthe range of −40° C. to 25° C. followed by addition ofpiperidinopiperidine to the cooled solution. More particularly, thephosgene trihydrate is cooled to a temperature in the range of −10° C.to 0° C.

In yet further embodiments, (1) subsequent to the addition ofpiperidinopiperidine, N,N-diisopropylethylamine (Hünig's base) ortriethylamine or an equivalent base is added to the cooled solutionand/or (2) the process further comprises adding a catalyst to theintermediate solution along with the SN-38 and the organic base.

In a second embodiment, the present invention provides a compositioncomprising a solvent and the reaction product of: (i) a compound offormula I:

wherein each of the ring atoms may be carbon, or any one, two or threeof the ring atoms may be nitrogen, and R^(a), R^(b) and R^(c) are thesame or different and independently represent one or more optionalnon-hydrogen substituents on each of rings A, B and C, (ii) an amine offormula R¹R²NH wherein R¹ and R² are the same or different andindependently represent organic groups, and (iii) phosgene or a reactiveequivalent thereof.

These and other aspects of the invention will be apparent upon referenceto the attached figures and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show the chemical structures of camptothecin 1 andvarious analogs and derivatives of camptothecin, specifically,irinotecan 2, topotecan 3, SN-38 4, exatecan 5, lurtotecan 6 and CKD-6027.

DETAILED DESCRIPTION OF THE INVENTION

I. Definitions

As used herein, the following terms have the following meanings.

“Alkyl” refers to a hydrocarbon structure having from 1 to 14 carbonatoms, wherein the carbons are arranged in a linear, branched, or cyclicmanner, including combinations thereof. Lower alkyl refers to alkylgroups of from 1 to 6 carbon atoms. Examples of lower alkyl groupsinclude methyl, ethyl, propyl, isopropyl, butyl, s- and t-butyl and thelike. “Cycloalkyl” is a subset of alkyl and includes cyclic hydrocarbongroups of from 3 to 14 carbon atoms. Examples of cycloalkyl groupsinclude cyclopropyl, cyclobutyl, cyclopentyl, norbornyl, adamantyl andthe like. When an alkyl residue having a specific number of carbons isnamed, all geometric isomers having that number of carbons are intendedto be encompassed; thus, for example, “butyl” is meant to includen-butyl, sec-butyl, isobutyl and t-butyl; propyl includes n-propyl andisopropyl.

“Alkenyl” refers to an alkyl group having at least one site ofunsaturation, i.e., at least one double bond.

“Alkynyl” refers to an alkyl group having at least one triple bondbetween adjacent carbon atoms.

“Alkoxy” and “alkoxyl” both refer to moieties of the formula —O-alkyl.Examples include methoxy, ethoxy, propoxy, isopropoxy, cyclopropyloxy,cyclohexyloxy and the like. Lower-alkoxy refers to groups containing oneto six carbons. The analogous term “aryloxy” refers to moieties of theformula —O-aryl.

“Alkanediyl” means a divalent alkyl from which two hydrogen atoms aretaken from the same, or different, carbon atoms, such as —CH₂—,—CH₂CH₂—, —CH₂CH₂CH₂—, —CH(CH₃)CH₂—, and the like.

“Acyl” refers to moieties of the formula —C(═O)-alkyl. One or morecarbons in the acyl residue may be replaced by nitrogen, oxygen orsulfur as long as the point of attachment to the parent remains at thecarbonyl. Examples include acetyl, benzoyl, propionyl, isobutyryl,t-butoxycarbonyl, benzyloxycarbonyl and the like. Lower-acyl refers togroups containing one to six carbons.

“Aryl” refers to an aromatic carbocyclic moiety such as phenyl ornaphthyl.

“Heteroalkyl” is a monovalent, saturated or unsaturated, straight orbranched, chain containing carbon and at least one heteroatom. Theheteroalkyl group may, in various embodiments, have one heteroatom, or1-2 heteroatoms, or 1-3 heteroatoms, or 1-4 heteroatoms. Heteroalkylchains may contain from 1 to 14 (i.e., 1-14) member atoms (carbon andheteroatoms) in the chain, and in various embodiments contain 1-12, or1-6, or 1-4 member atoms. Independently, in various embodiments, theheteroalkyl group has zero branches (i.e., is a straight chain), onebranch, two branches, or more than two branches. Independently, in oneembodiment, the heteroalkyl group is saturated. In another embodiment,the heteroalkyl group is unsaturated. In various embodiments, theunsaturated heteroalkyl may have one double bond, two double bonds, morethan two double bonds, and/or one triple bond, two triple bonds, or morethan two triple bonds.

“Heteroaryl” refers to a 5- or 6-membered heteroaromatic ring containing1-3 heteroatoms selected from O, N, or S; a bicyclic 9- or 10-memberedheteroaromatic ring system containing 0-3 heteroatoms selected from O,N, or S; or a tricyclic 13- or 14-membered heteroaromatic ring systemcontaining 0-3 heteroatoms selected from O, N, or S. Exemplary aromaticheterocyclic rings include, e.g., imidazole, pyridine, indole,thiophene, benzopyranone, thiazole, furan, benzimidazole, quinoline,isoquinoline, quinoxaline, pyrimidine, pyrazine, tetrazole and pyrazole.

“Heterocycle” means a 5- to 7-membered monocyclic, or 7- to 10-memberedbicyclic, heterocyclic ring which is either saturated, unsaturated, oraromatic, and which contains from 1 to 4 heteroatoms independentlyselected from nitrogen, oxygen and sulfur, and wherein the nitrogen andsulfur heteroatoms may be optionally oxidized, and the nitrogenheteroatom may be optionally quaternized, including bicyclic rings inwhich any of the above heterocycles are fused to a benzene ring. Theheterocycle may be attached via any heteroatom or carbon atom.Heterocycles include heteroaryls as defined above. Thus, in addition tothe heteroaryls listed above, heterocycles also include morpholinyl,pyrrolidinonyl, pyrrolidinyl, piperidinyl, piperazinyl, hydantoinyl,valerolactamyl, oxiranyl, oxetanyl, tetrahydrofuranyl,tetrahydropyranyl, tetrahydropyridinyl, tetrahydroprimidinyl,tetrahydrothiophenyl, tetrahydrothiopyranyl, tetrahydropyrimidinyl,tetrahydrothiophenyl, tetrahydrothiopyranyl, and the like.

The term “substituted” as used herein means any of the above groups(e.g., alkyl, alkoxy, acyl, aryl, heteroalkyl, heteroaryl andheterocycle) wherein at least one hydrogen atom is replaced with asubstituent. In the case of an oxo substituent (“═O”) two hydrogen atomsare replaced. Substituents include halogen, hydroxy, oxo, alkyl,substituted alkyl, aryl, substituted aryl, heteroalkyl, substitutedheteroalkyl, heteroaryl, substituted heteroaryl, heterocycle,substituted heterocycle, —NR_(a)R_(b), —NR_(a)C(═O)R_(b),—NR_(c)C(═O)NR_(a)R_(b), —NR_(a)C(═O)OR_(b), —NR_(a)SO₂R_(b), —OR_(a),—C(═O)R_(a), C(═O)OR_(a), —C(═O)NR_(a)R_(b), —OC(═O)R_(a),—OC(═O)OR_(a), —OC(═O)NR_(a)R_(b), —NR_(a)SO₂R_(b), or a radical of theformula —Y-Z-R_(a) where Y is alkanediyl, substituted alkanediyl or adirect bond, Z is —O—, —S—, —S(═O)—, —S(═O)₂—, —N(R_(b))—, —C(═O)—,—C(═O)O—, —OC(═O)—, —N(R_(b))C(═O)—, —C(═O)N(R_(b))— or a direct bond,wherein R_(a), R_(b) and R_(c) are the same or different andindependently hydrogen, amino, alkyl, substituted alkyl (includinghalogenated alkyl), aryl, substituted aryl, heteroalkyl, substitutedheteroalkyl, heteroaryl, substituted heteroaryl, heterocycle, orsubstituted heterocycle, or wherein R_(a) and R_(b) taken together withthe nitrogen atom to which they are attached form a heterocycle orsubstituted heterocycle.

“Formyl” refers to the moiety —C(═O)H.

“Halogen” refers to fluoro, chloro, bromo or iodo.

II. Overview of the Invention

As noted above the present invention provides synthetic methods andcompounds produced by, or using, the synthetic methods. The compoundsare useful as synthetic intermediates in the preparation of derivativesand analogs of irinotecan, where the synthetic intermediates may alsohave desirable biological activity.

In one aspect, the present invention provides a process for adding asidechain to a starting material, comprising reacting together:

(i) a compound of formula I:

wherein each of the ring atoms may be carbon, as shown, or any one, twoor three of the ring atoms may be nitrogen, and R^(a), R^(b) and R^(c)are the same or different and independently represent one or moreoptional non-hydrogen substituents on each of rings A, B and C,respectively,

(ii) an amine of formula R¹R²NH wherein R¹ and R² are the same ordifferent and independently represent organic groups, and

(iii) phosgene or a reactive equivalent thereof,to provide a compound of formula III:

Each of the three reactants, namely, (i) a compound of formula I:

(ii) an amine of formula R¹R²NH, and (iii) phosgene or a reactiveequivalent thereof, will now be described.Compound of Formula I

In the structure provided above for compounds of formula I, each of thering atoms is shown as carbon. However, this depiction is for purposesof illustration only, and is intended to encompass compounds which, inaddition to being carbocyclic may have, with one exception, any one, twoor three of the ring atoms replaced with nitrogen. The one exception isthat the ring atom at the C10 position must be carbon.

Thus, in one aspect, one of the ring carbon atoms is replaced withnitrogen and representative compounds of formula I have the followingstructures:

In another aspect, two of the ring carbon atoms are replaced withnitrogen and a representative compound of formula I may have thefollowing structure:

In yet another aspect, three of the ring carbon atoms may be replacedwith nitrogen. In this regard, any three carbon atoms except the carbonatom at the C10 position may be replaced with nitrogen.

In formula I, only the ring atom at the C10 position is specificallyshown to be substituted with a non-hydrogen substituent. However, asfurther noted above, formula I is defined herein to encompass compoundswherein any one or more of the A, B and C rings may be substituted withone or more optional non-hydrogen substituents. This feature of thecompounds of formula I is shown by the designations R^(a), R^(b) andR^(c), where R^(a) represents one or more optional non-hydrogensubstituents on the A ring, and R^(b) and R^(c) have analogousdefinitions for the B and C rings, respectively. Furthermore, formula Iis defined herein to encompass compounds wherein two R^(a) substituents,two R^(b) substituents and/or two R^(c) substitutents, together with thering atoms to which they are attached, may optionally be joined togetherto form a carbocyclic or heterocyclic ring. In various embodiments,R^(a), R^(b) and R^(c) have, in total, no more than 30 carbon atoms andno more than 5 oxygen atoms.

Representative R^(a), R^(b) and R^(c) groups include alkyl, substitutedalkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl,alkoxy, substituted alkoxy, acyl, substituted acyl, aryl, substitutedaryl, heteroalkyl, substituted heteroalkyl, heteroaryl, substitutedheteroaryl, heterocycle, substituted heterocycle, formyl and halogen.

For example, in one aspect of the present invention, no optional R^(a)substituents are present, i.e., the A ring has no substitutions beyondthe hydroxy group at the C10 position. In another aspect, R^(b)represents an alkyl group at the C7 position as the only optionalsubstituent on the B ring. In yet another aspect, R^(c) represents twooptional substituents on the C ring which are joined together, inparticular, R^(c) represents the well-known D and E rings of SN-38,i.e., the compound of formula I is substituted with D and E rings asshown below:

Accordingly, in one aspect of the present invention, the startingmaterial used in the process of the present invention has the formulaII, as follows:

wherein R³ is hydrogen or an alkyl group, and each of the ring atoms iscarbon or nitrogen as shown. In one aspect, R³ is ethyl. In anotheraspect, R³ is hydrogen. In another aspect, R³ is a C₁-C₆ alkyl group.The compounds of formula II are particularly similar to SN-38, and thusare a typical starting material. However, the process of the presentinvention is more generally applicable to compounds of formula I.Amine of Formula R¹R²NH

A second reactant in the process of the present invention is an amine ofthe formula R¹R²NH. As for groups R¹ and R², these represent organicgroups, where in various embodiments of the invention, R¹ and R² have,in total, less than 20 non-hydrogen atoms, or less than 15 non-hydrogenatoms.

Representative R¹ and R² groups include alkyl, substituted alkyl,alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, alkoxy,substituted alkoxy, acyl, substituted acyl, aryl, substituted aryl,heteroalkyl, substituted heteroalkyl, heteroaryl, substitutedheteroaryl, heterocycle and substituted heterocycle.

The R¹ and R² groups may be separate from one another, e.g., as in thestructures:

or the R¹ and R² groups may be joined together, e.g., as in thestructures:

When the R¹ and R² groups are joined together, the heterocyclic ringthat is thereby formed may optionally be substituted, e.g., as in thestructures:

An amine of the structure

is a typical compound of formula R¹R²NH according to the presentinvention because this amine duplicates the sidechain present inirinotecan.Phosgene or a Reactive Equivalent Thereof

In addition to the compounds of formula I and amines of the formulaR¹R²NH, the third reactant that is employed in the process of theinvention is phosgene or a reactive equivalent thereof. Examples ofreactive equivalents of phosgene include phosgene dimer (also known astrichloromethyl chloroformate or diphosgene), phosgene trimer (alsoknown as triphosgene or bistrichloromethyl carbonate),carbonyldiimidazole and a combination of di-tert-butyl carbonate (DIBOC)with 4-(N,N-dimethylamino)pyridine (DMAP). Phosgene and the reactiveequivalents thereof may be in solid (e.g., phosgene trihydrate), liquidor gaseous form.

In the method of the present invention, a compound of formula I, acompound of formula R¹R²NH, and phosgene or a reactive equivalentthereof are reacted. These three reactants are combined in a singlereaction vessel. The order of addition is not critical. In one aspect,the phosgene compound is added to the vessel, followed by addition ofthe R¹R²NH compound, followed by addition of the formula I compound.However, other orders of combination may also be employed.

A solvent will also typically be present in the reaction vessel.Exemplary solvents include halogenated solvents (e.g., dichloromethaneand chloroform), ethereal solvents (e.g., diethyl ether andtetrahydrofuran), and glymes.

In addition to the solvent, a base is also typically present. The baseis thought to act by activating the hydroxy group at the C10 position ofthe compound of formula I so that it is more reactive and will convertto the desired sidechain moiety. In one aspect, the base is anitrogen-containing compound, e.g., pyridine, DABCO, tertiary amines andthe like. A catalytic amount of a condensing agent can also be added,such as DMAP or DCC and the like.

The reaction temperature is typically between about −25° C. and +25° C.,however other temperatures may be employed. The temperature primarilyaffects the rate of reaction, and may be adjusted to achieve a reactionrate that is convenient for the operator.

After the reactants are combined, they are left in combination for asufficient time to form the side chain at C10. This time is typically onthe order of 30 minutes to 5 hours when the reaction temperature is heldbetween −10 and 0° C. The reaction may be conducted under an inertatmosphere, e.g., nitrogen or argon. The reaction vessel contents aretypically stirred or otherwise agitated.

In one aspect of the invention, the compound of formula 1 and the amineof formula R¹R²NH are, respectively, SN-38, having the followingstructure:

piperidinopiperidine, having the structure:

In the foregoing embodiment, when the compound of formula I is SN-38,and the amine of formula R¹R²NH is piperidinopiperidine, a compound offormula III is formed, namely, irinotecan, having the followingstructure III-A:

The product mixture obtained by combining a compound of formula I, acompound of formula R¹R²NH, and phosgene or reactive equivalent thereofmay be subjected to a purification scheme to obtain a compound offormula III in relatively pure form. For example, the product mixturemay be filtered and the supernatant subjected to a solvent removalprocess (e.g., rotary evaporation). The residue may then be furtherpurified by column chromatography, precipitation or crystallization. Forexample, the residue may be dissolved in a solvent, e.g.,dichloromethane, and applied to a silica gel column. Elution with agradient or mixture of dichloromethane and methanol affords a purifiedcompound of formula III.

The process of the present invention wherein a sidechain is added to acompound of formula I is very efficient. This one-pot procedure mayyield a final product of formula III in greater than 50% molar yield,based on the moles of compound of formula I employed. In furtheraspects, the molar yield is greater than 75% based on the moles ofcompound of formula I used in the process. In yet further aspects, themolar yield is greater than 90% based on the moles of compound offormula I used in the process. In a specific embodiment, the molar yieldis 94% based on the moles of compound of formula I used in the process.

In those instances where the product is a basic, nitrogen-containingcompound, then a further embodiment of the present invention provides apurification process for the product wherein the product is convertedinto a salt form. Such a purification process includes combining aproduct mixture comprising a nitrogen-containing compound (e.g.,irinotecan) with a chlorinated solvent (e.g., dichloromethane (DCM)) oran aqueous solvent (e.g., water) and cooling this mixture to about 0° C.This solution is then acidified by adding an acid (e.g., 12N HCl) dropwise with stirring over a period of 2-3 hours. The solvent is evaporatedto leave a solid residue which is dissolved in an alcoholic solvent(e.g., ethanol or methanol) and precipitated by addition of an ethersolvent (e.g., diethyl ether). The precipitate is washed with the ethersolvent to afford the pure salt.

The present invention is further illustrated by the followingnon-limiting examples. Unless otherwise noted, all scientific andtechnical terms have the meanings as understood by one of ordinary skillin the art.

EXAMPLES Example 1

Phosgene trihydrate was dissolved in a chlorinated solvent, such as DCMunder an argon atmosphere and cooled down to a low temperature in therange of −10 to 0° C. To this solution was added sequentially,piperidinopiperidine in DCM drop wise, followed by the addition ofN,N-diisopropylethylamine (Hünig's base) or TEA drop wise. The reactionmixture was stirred at this temperature for one hour and slowly warmedto around room temperature and kept at this temperature for 2 hours.

After this time, a solution of SN-38 in pyridine was added drop wise andthe solution was left to react for 30 minutes to 2 hours or until thecomplete consumption of starting material as evidenced by TLC.

The reaction was filtered and concentrated under vacuum to get the crudeproduct. The crude product was dissolved in DCM and washed with water,dried over anhydrous Na₂SO₄ or MgSO₄ and evaporated. This material waspurified either by column chromatography using silica gel and elutedwith mixture of DCM/MeOH or precipitation or crystallization to obtainthe pure product, namely, irinotecan (i.e., the compound of formulaIII-A). The yield of the desired product is 94%.

Example 2

The product from Example 1 was dissolved in DCM and cooled down to 0° C.12N HCl was added drop wise to this solution and stirred for 2-3 hours.The solvent was evaporated to get a solid which was dissolved inmethanol and precipitated by addition of diethyl ether. The precipitateis washed by ether 3 times to afford the pure product, namely, the saltform of irinotecan (i.e., the compound of formula III-A). The yield ofthe desired product is quantitative.

Alternatively, the product from Example 1 is dissolved in water and 12NHCl is added drop wise to the solution. The product precipitates out andis obtained by filtration. The yield is 95%.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing, it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A process for adding a sidechain to a starting material comprisingreacting together: (i) a compound of formula I:

wherein each of the ring atoms may be carbon, or any one, two or threeof the ring atoms may be nitrogen, and R^(a), R^(b) and R^(c) are thesame or different and independently represent one or more optionalnon-hydrogen substituents on each of rings A, B and C; (ii) an amine offormula R¹R²NH wherein R¹ and R² are the same or different andindependently represent organic groups; and (iii) phosgene or a reactiveequivalent thereof, to provide a solution comprising a compound offormula III:


2. The process of claim 1 wherein R^(a), R^(b) and R^(c) have, in total,no more than 30 carbon atoms and no more than 5 oxygen atoms.
 3. Theprocess of claim 1 wherein the compound of formula I has the structure:

wherein R³ is an alkyl group.
 4. The process of claim 3 wherein thecompound of formula I has the structure:

or a partially or completely purified stereoisomer thereof, or a saltthereof.
 5. The process of claim 1 wherein the amine of formula R¹R²NHis piperidinopiperidine.
 6. The process of claim 5 wherein the compoundof formula I has the structure:

wherein R³ is an alkyl group.
 7. The process of claim 1 wherein thephosgene or a reactive equivalent thereof is phosgene trihydrate.
 8. Theprocess of claim 1 wherein the phosgene or reactive equivalent thereofand the amine of formula R¹R²NH are combined to provide an intermediatesolution, and the compound of formula I is added to the intermediatesolution.
 9. The process of claim 8 further comprising adding pyridineor a tertiary amine to the intermediate solution along with the compoundof formula I.
 10. The process of claim 1 further comprising a stepwherein the solution comprising a compound of formula III is filteredand then freed from solvent to provide a residue.
 11. The process ofclaim 10 wherein the residue is purified by column chromatography,filtration, precipitation or crystallization.
 12. The process of claim 1wherein the molar yield of the compound of formula III is at least 50%based on the moles of the compound of formula I used in the process. 13.The process of claim 1 wherein the molar yield of the compound offormula III is at least 75% based on the moles of the compound offormula I used in the process.
 14. The process of claim 1 wherein themolar yield of the compound of formula III is at least 90% based on themoles of the compound of formula I used in the process.
 15. The processof claim 1 wherein the molar yield of the compound of formula III is 94%based on the moles of the compound of formula I used in the process. 16.The process of claim 1 wherein the compound of formula I is SN-38, theamine of formula R¹R²NH is piperidinopiperidine, and the compound offormula III is irinotecan, and wherein the process is a one-potoperation.
 17. The process of claim 16 wherein the phosgene or reactiveequivalent thereof is phosgene trihydrate, and the phosgene trihydrateand piperidinopiperidine are combined in a solvent and allowed to reacttogether to form an intermediate solution, and the SN-38 and an organicbase are added to the intermediate solution to form the solutioncomprising irinotecan.
 18. The process of claim 17 wherein the phosgenetrihydrate is dissolved in a chlorinated solvent and then cooled to atemperature in the range of −40° C. to 25° C. followed by addition ofpiperidinopiperidine to the cooled solution.
 19. The process of claim 18wherein the phosgene trihydrate is cooled to a temperature in the rangeof −10° C. to 0° C.
 20. The process of claim 18 wherein, subsequent tothe addition of piperidinopiperidine, N,N-diisopropylethylamine,triethylamine or an equivalent base is added to the cooled solution. 21.The process of claim 20 further comprising adding a catalyst to theintermediate solution along with the SN-38 and the organic base.
 22. Theprocess of claim 21 wherein the organic base is pyridine or the like.23. The process of claim 21 wherein the catalyst is DMAP, DCC or thelike.
 24. A composition comprising a solvent and the reaction productof: (i) a compound of formula I:

wherein each of the ring atoms may be carbon, or any one, two or threeof the ring atoms may be nitrogen, and R^(a), R^(b) and R^(c) are thesame or different and independently represent one or more optionalnon-hydrogen substituents on each of rings A, B and C, (ii) an amine offormula R¹R²NH wherein R¹ and R² are the same or different andindependently represent organic groups, and (iii) phosgene or a reactiveequivalent thereof.
 25. The composition of claim 24 wherein the compoundof formula I is SN-38 and the amine of formula R¹R²NH ispiperidinopiperidine.